Tridacnid and Coral Bleaching, Part 1: What It Is and What Causes It

Author: James W. Fatherree, MSc

Photographer: James Fatherree

The Reefer: January 2007

Under certain adverse conditions, corals that contain symbiotic algae (zooxanthellae) may become pale in color, or even turn completely white. This condition is appropriately called bleaching, and it can affect small areas of a coral, or at times a whole individual or colony. The giant (tridacnid) clams also depend on populations of zooxanthellae for the bulk of their nutritional needs, and they too can suffer from bleaching. While both corals and clams may recover from this condition in the wild, they often don’t in aquariums unless action is taken, so it is important to understand what bleaching is and what to do if it happens in your tank. This month we’ll take a look at the types of bleaching and what can cause them, and in Part 2 I’ll go over some of the things you can do to try to reverse the process if it happens in your aquarium.

What is Bleaching?

In the most basic sense, bleaching is simply a lightening in color of a tridacnid clam or coral that results from the loss of zooxanthellae and the various pigments associated with them. The colors of these creatures that we see come from various pigments in the zooxanthellae and other pigments that the host animal produces, so when these things are reduced in quantity, a host will lighten up/fade out. There are several things that can cause this to occur to some degree or another, but based on my experience I’d say that over-illumination (light shock) and/or unacceptably high temperatures are the main problems in aquariums. These are also two primary causes of bleaching in the wild.

Tridacnids and corals can protect themselves from over-illumination in a number of ways, and they can adapt to increased lighting over time. But, if the amount of light (especially UV light) that one of them receives increases faster than the organism can adapt to the change, the result can be cellular damage and an unhealthy overload of the photosynthetic process carried out by the zooxanthellae; I don’t mean that they start to make more food than the host can handle, either.

Instead of providing their host animal with nutritious substances and “normal” oxygen, light-shocked zooxanthellae can produce some nasty chemicals called active oxygen radicals, which can further interfere with the photosynthetic process and lead to serious troubles within them. The host can lose its primary source of food if this happens, and the zooxanthellae can also be outright fried. Unacceptably high temperatures can also throw a monkey wrench into the photosynthetic process, which can make things even worse. So something has to change when light intensity and/or temperatures go up too much (or just too fast in the case of light) or the host and the zooxanthellae can both end up dead.

When the lights get too bright, and/or the temperatures get too high, a reduction in the number of zooxanthellae that corresponds to the severity of the situation is the typical result. In different situations it isn’t clear if a host actually expels the zooxanthellae, digests them and uses them for food (since they probably can’t make enough anymore), or if they just die faster than they can reproduce/be replaced. However, the end effect is always the same: a reduced population of zooxanthellae within the affected host. It may be only a small decrease in numbers in some situations, but in most cases the size of the population may drop dramatically. In fact, almost every last one of them may disappear, leaving their host just about empty. That’s only in extreme cases though, as there is usually at least a small population of zooxanthellae that’s held onto, which can later begin to reproduce and repopulate a host’s tissues if conditions improve.

So how does this sort of thing occur in the wild? In reef environments around the world, the length of days, the average daily cloud cover, and water temperatures typically change very slowly through the seasons. However, there are times when temperatures increase more than usual, and there are times when decreased wind/wave activity allows more light to penetrate deeper into waters, both of which can lead to bleaching.

Wave activity can break up incoming sunlight to some degree. It can also keep sediments stirred up at least a bit in shallow waters, which also block some incoming sunlight. But on the other hand, no wind means no waves, and no waves means more light making it through the surface while the sediments simultaneously settle to the bottom and let more light fall upon whatever is below. This can lead to light shock and then to bleaching as more than the normal dose of light reaches tridacnids and corals. Likewise, bleaching can occur in an aquarium if a clam is placed under aquarium lights that are more intense than the lighting they’re used to without being given adequate time to adapt to them, and/or if the temperature ever gets too high.

This is why you absolutely must give tridacnids and corals time to adapt to your lighting system unless you are positive that your lights are dimmer than what the specimen is already used to. The same goes for temperature; you should never run your tank at an unacceptably high temperature or allow it to spike if the air conditioner should ever fail, etc.

What Else Causes It?

On the other hand, tridacnids and corals may also bleach as a response to under-illumination and/or a lack of nutrients. This can also happen both in tanks and on the reef. If there isn’t enough light to promote photosynthesis and/or a specimen can’t get enough nitrogen, phosphorous, etc. the zooxanthellae may simply starve to death, leaving a host bleached. Unacceptably low temperatures can have the same effect, as there’s only so low that they can go and still function properly. However, as long as you allow a specimen to acclimate to tank conditions over time, keep things at an acceptable temperature, and provide it with enough light and nutrients, the chances that bleaching will strike are slim. Slim is not the same as zero, though, and there are yet still more things that may bring it on.

It has been shown that unacceptable salinities (too low or high), poisoning by metals, too much red light, the use of some medications, and even general stress can be factors in bleaching (ex. Duquesne & Coll 1995, Brown 1997, Braley 1998, and Borneman 2001), and some microbial infections may bring it on as well.

Disease-related bleaching has been reported to occur in corals (Kushmaro et al. 1996 and Rosenberg & Loya 1999), and was also suggested by Norton et al. (1995) as a probable candidate for some cases of tridacnid bleaching. However, while the bacteria Vibrio shiloi was positively identified as the pathogen in the case of corals, no specific bacteria or other microorganism has been identified as the cause of clam bleaching. Still, there’s strong evidence that it is possible, so something specific may be found in the future.

Additionally, some cases of bleaching may be the result of a clam’s inability, when kept in an aquarium, to produce something that they and the zooxanthellae require in order to function properly (Knop 1996). In other words, for some unknown reason, a host may be unable to produce sufficient quantities of UV-blocking substances. Or maybe it’s the zooxanthellae that can’t. The zooxanthellae do produce a number of pigments, but they’re also the likely source of the substances that a host uses in order to produce other pigments itself (Borneman 2001). So, if things go wrong for either or both partners, these sunscreens may be difficult or impossible to make. That sounds reasonable enough to me.

As you can see, unfortunately there are many things can cause bleaching to occur.

What Are the Different Types?

Bleaching can generally be seen in three different forms, one of which affects tridacnids only. There’s the generalized bleaching and localized bleaching that affect corals and clams, then there’s central bleaching, which affects only clams. Let’s look at each of these and why they’re thought to occur.

Generalized bleaching is seen as a relatively uniform loss of color over the entire mantle of a tridacnid (the soft, extendable tissue that protrudes from the shell), or a single coral or coral colony. It can range in severity from a slight lightening to a complete loss of color, and it’s the most likely to lead to a specimen’s death. This is what’s most likely to occur as a result of light shock, elevated temperatures, too little light or nutrients (starvation), too high or low salinity, or some sort of poisoning; essentially anything that affects the whole specimen.

Localized bleaching is quite different and can be seen as patches of lightening that may occur anywhere on a specimen. Again, there may be only a slight lightening to a complete loss of color, but in this case I doubt that unacceptable lighting or temperatures, or poisoning, etc. have anything to do with it. Instead, localized bleaching may be the result of an infection of some sort, as mentioned above, or some type of damage to a restricted area of a host’s tissues. It may also occur if a portion of the specimen is heavily shadowed by an overhang, a coral branch, or some other structure.

If it occurs as a reaction to being shadowed, it should remain restricted roughly to the area that isn’t getting any light. But if it is due to an infection (or something else), it may start as a small area and then spread over larger portions of the specimen. Still, in some cases it shows up in one spot for no apparent reason and doesn’t spread much, if at all, as I’ve seen occur many times on tridacnid mantles.

As long as it doesn’t spread over much of the specimen, localized bleaching doesn’t seem to have any noticeable effect on the overall fitness of tridacnids, and it is often seen on otherwise perfectly healthy clams. I’ve seen numerous specimens with small areas of bleaching that still grew normally, and they didn’t seem to be bothered at all. Oddly enough, it tends to be permanent too, at least for clams. In the case of corals, it also may persist, spread, or go away over time.

Central bleaching is unique to tridacnids though, and it’s seen as a loss of color in the central area of the upper mantle, particularly on any relatively flat portions of it, which can also range in severity from a slight lightening to a complete loss of color. This is most likely the result of over-illumination, as only part of a specimen is affected, and it happens to be the part that receives light in a perpendicular fashion even if the mantle is not fully extended. This area is exposed to incoming light even if the shell is only partially open and the mantle is retracted. However, Knop (1996) suggests that it may be due to a reduced ability to produce sunscreens, as mentioned above. Central bleaching is far less likely to cause the death of a clam. However, if action isn’t taken to correct to situation that brought it on, it may spread over the rest of the mantle, becoming generalized bleaching.

As a last note, be aware that many tridacnids have areas of their mantles that are devoid of color and look rather translucent, but this is not the result of bleaching. Specimens of Tridacna gigas often have a substantial amount of color-free mantle tissue, and T. derasa sometimes has colorless round spots dotted over its mantle. Really, all of the species can have some colorless areas, particularly in the central area of the mantle, so don’t automatically jump to conclusions if you happen to see any light spots. Such areas may not be signs of trouble. The same goes for branch tips on many stony corals, as well. It is common for the very ends of the branches of an Acropora specimen, or some other small-polyp coral, to be white in color, but again, in this case it’s normal. However, if you see an obvious overall lightening or increases in the size and/or abundance of colorless areas over a relatively short period of time, then you do need to be worried and must take action.